Pressure sensor
A pressure sensor including: A pressure measuring cell having an end face loadable with the medium; a housing having a media opening and a ring-shaped, axial bearing surface, which surrounds the media opening; a clamping apparatus; and a ring-shaped sealing arrangement. The pressure measuring cell is positioned in the housing and the sealing arrangement is positioned between the bearing surface and the end face, and the sealing arrangement, as well as the pressure measuring cell, are axially clamped between the bearing surface and the clamping apparatus. The sealing arrangement includes, a decoupling ring, as well as a first and a second, ring-shaped sealing element. The first sealing element lies against the end face, the second sealing element lies against the bearing surface, and the decoupling ring is axially clamped between the first and the second sealing elements.
Latest Endress + Hauser GmbH+Co. KG Patents:
- Measuring arrangement with a pressure sensor module
- Method and system for remote-controlled servicing of a field device of process automation
- Radio module for field devices of automation technology
- Dispersion correction for FMCW radar in a pipe or tube
- Method for calibration or adjustment of any oscillatable unit
The present invention relates to a pressure sensor for measuring pressure of a medium.
Especially in the case of pressure sensors that are used in aggressive chemicals, the sealing between the pressure measuring cell and the media opening of the sensor housing proves to be problematic, since the usual elastic O-rings either can not be used, or can be used only with limitations. Instead, PTFE seals are applied. While PTFE has the desired chemical stability, it is accompanied by mechanical problems. Thus, PTFE does not have sufficient elasticity and it even flows under pressure. Kathan et al. disclose in Publication DE 19628255 A1 a pressure measuring device, in which a ring-shaped gasket of PTFE is axially clamped between the media-facing surface of the pressure measurement cell and an axial bearing surface of the sensor housing, with the axial bearing surface having resilient properties. This attempted solution leads, however, to temperature hysteresis errors, since the coefficient of thermal expansion of the steel housing differs greatly from that of the ceramic measuring cell and this directly affects the measuring membrane of the measuring cell. Flogel et al. manage, to some extent, to provide a remedy in the teachings of Publication DE 101 06 129 A1, for the sensor proposed there has, in the region of the media opening of the housing, a relatively massive Kovar ring, which supports the axial bearing surface, whereby an approximation of the effective coefficient of thermal expansion of the axial bearing surface is supposed to be obtained. Although this perhaps achieves the desired approximation of the coefficient of thermal expansion, now temperature stresses can be expected between the steel parts of the housing and the Kovar ring, which can still get back to affect the measuring cell.
An object of the invention is, therefore, to provide a pressure sensor, which overcomes the recited disadvantages of the state of the art. This object is solved according to the invention by the pressure sensor as defined in independent claim 1.
The pressure sensor of the invention includes: A pressure measuring cell having an end face loadable with the medium; a housing having a media opening and a ring-shaped, axial, bearing surface, which surrounds the media opening; a clamping apparatus; and a ring-shaped sealing arrangement; wherein the pressure measuring cell is positioned in the housing and the sealing arrangement is positioned between the bearing surface and the end face, and the sealing arrangement, as well as the pressure measuring cell, are axially clamped between the bearing surface and the clamping apparatus; characterized in that the sealing arrangement includes a decoupling ring as well as first and second, ring-shaped, sealing elements, the first sealing element lies against the end face, the second sealing lies against the bearing surface, and the decoupling ring is axially clamped between the first and second sealing elements.
The term “ring-shaped” is meant to include not only circular rings, but, also, any ring having a closed path around an opening. These can have, among others, also an oval, rectangular, hexagonal, or any other polygonal shape.
The pressure measuring cell can include a platform and a measuring membrane, or diaphragm, of a first material, which can be, for example, a ceramic, especially corundum, or a crystalline material. The decoupling ring should, preferably, be manufactured from a material, whose mechanical and/or thermal properties essentially approximate, or equal, those of the first material. This can be achieved, for example, by manufacturing the decoupling ring likewise from the first material.
The decoupling ring is preferably so embodied that radial forces, which can arise between the decoupling ring and the axial bearing surface due to thermal expansion differences, lead, in any case, only to negligible deformations of the decoupling ring. This can be achieved, for example, by a certain stiffness, and, on the other hand, by the use of a decoupling ring having two planparallel end faces. In the way, thermal expansion differences between the axial bearing surface and the decoupling ring essentially lead exclusively to shear forces, which are, to the greatest possible extent, relieved by deformations of the second sealing element.
It is permissible to provide projections and/or cavities on the end faces, which can be, for example, ring-shaped, for the purpose of limiting flow of the sealing elements under load. This is especially to be considered, when the first and/or the second sealing element(s) involve(s) a material such as PTFE, especially in form the form of a flat seal, i.e. a gasket. In some cases, it can also be advisable to use O-rings as sealing elements. In this case, in each instance, at least one of the end faces, between which the O-rings are clamped, should have the requisite form of an O-ring bed.
In a currently preferred embodiment of a pressure sensor of the invention, the clamping apparatus and/or the housing includes an axially elastic element. This axially elastic element can be, among other things, a Belleville spring, which forms, or is integrated in, for example, the axial bearing surface, a corrugated tube membrane, or diaphragm, integrated into the media opening, as well as a spring, e.g. a helical spring, or a Belleville spring, axially clamped between the rear side of the pressure measuring cell and a clamping ring.
The elastic element assures sufficient elasticity that the sealing elements are, in the case of pressure fluctuations, and pressure shocks, or spikes, of the medium, as well as in the case of temperature fluctuations of the pressure sensor, only subjected to such fluctuations of the axial clamping pressure, as do not compromise the sealing effect. Additionally, the elastic element serves to compensate for settling, or plastic deformation, of the sealing elements under load. The elasticity is preferably so selected that the axial clamping pressure on the first and second sealing elements does not, over temperature cycles between −40° C. and 150° C., fluctuate by more than 50%, more preferably by more than 25%, and especially preferably by not more than 12%, of the maximum occurring clamping pressure.
Furthermore, it is currently preferred that the axial clamping pressure on the first and second sealing elements does not, over temperature cycles between −40° C. and 150° C., sink below 0.8 MPa, preferably below 0.9 MPa, and especially preferably below 1 MPa.
The invention will now be described on the basis of the appended drawings, the figures of which show as follows:
The pressure sensors shown in FIGS. 14 include a cylindrical, ceramic, pressure measuring cell, for example of corundum, operating using the capacitive measurement principle. Of course, also any other pressure measuring cells can be used, for example those utilizing pressure-dependent resonators or resistors. The pressure measuring cells are, in the illustrated examples of embodiments, in each case, axially clamped, in different ways, in a housing.
The pressure sensor shown in
Sealing arrangement 30 comprises essentially a decoupling ring 33, which is made of a ceramic material, especially corundum. The decoupling ring 30 includes, preferably, two planparallel, end faces 31, 32, by means of which a first ring-shaped sealing element 31 and a second ring-shaped sealing element 32 are pressed, respectively, against the pressure measuring cell, and against the axial bearing surface 22. The end faces can have structural elements for limiting the radial movement of the sealing elements 31, 32, especially the cold flow of PTFE sealing elements under pressure. Currently preferred as sealing elements are flat seals, i.e. gaskets, especially gaskets of PTFE. Preferably, the radial extent of the gaskets, thus the distance between the inner and outer radii thereof, amounts to greater than ten-times the gasket material thickness in the axial direction.
The clamping force for the PTFE gaskets is, for example, so selected that, over the entire temperature range of the pressure sensor, the axial clamping pressure does not subceed, or fall below, 0.8 MPa, preferably 0.9 MPa and especially preferably 1 MPa.
The pressure measuring cell in
In the case of the example of an embodiment shown in
The pressure sensor has a housing 120 with an axially stiff, radially inwardly extending shoulder, which bounds a media opening 121. Formed on the shoulder is an axial bearing surface 122, against which a pressure measuring cell with a platform 10 and a measuring membrane 11 is axially clamped by means of a threaded ring 123. A sealing arrangement 30 is, as above described, clamped between the pressure measuring cell and the axial bearing surface 122.
The axial elasticity for assuring a sufficiently constant clamping force is, in this case, assured by an elastic element, for example a spring ring 124 or a Belleville spring, with the elastic element being clamped between the rear-side of the pressure measuring cell and the threaded ring 123. In the dimensioning of the elastic element, a design consideration is that the elastic element must absorb overloading pressure shocks, without the seals being excessively unloaded.
The example of an embodiment shown in
The example of an embodiment shown in
Claims
1-14. (canceled)
15. A pressure sensor for measuring a pressure of a medium, comprising:
- A pressure measuring cell having an end face loadable with the medium;
- a housing having a media opening and a ring-shaped, axial bearing surface surrounding the media opening;
- a clamping apparatus; and
- a ring-shaped sealing arrangement; wherein
- said pressure measuring cell is positioned in said housing and said sealing arrangement is positioned between said bearing surface and said end face, said sealing arrangement, as well as said pressure measuring cell are axially clamped between said bearing surface and said clamping apparatus; and
- said sealing arrangement includes a decoupling ring as well as a first, and a second, ring-shaped sealing element, said first sealing element lies against said end face, said second sealing element lies against said bearing surface, and said decoupling ring is axially clamped between said first sealing element and said second sealing element.
16. The pressure sensor as claimed in claim 15, wherein:
- said pressure measuring cell has a platform, and a measuring membrane, of a first material, said decoupling ring is made of a second material; and the mechanical and/or thermal properties of said first material equal those of said second material.
17. The pressure sensor as claimed in claim 16, wherein:
- said first material and said second material are equal.
18. The pressure sensor as claimed in claim 16, wherein:
- said first material comprises a ceramic, especially corundum, or a crystalline material.
19. The pressure sensor as claimed in claim 16, wherein:
- said first and/or said second sealing element comprise(s) an inert material, especially PTFE.
20. The pressure sensor as claimed in claim 15, wherein:
- said decoupling ring is bounded in the axial direction by two planparallel end faces.
21. The pressure sensor as claimed in claim 20, wherein:
- said end faces have ring-shaped projections and/or cavities.
22. The pressure sensor as claimed in claim 15, wherein:
- said clamping apparatus and/or said housing includes an axially elastic element.
23. The pressure sensor as claimed in claim 27, wherein:
- the axial clamping pressure on said first and second sealing elements fluctuates over temperature cycles between −40° C. and 150° C. by not more than 40%, preferably by not more than 20% and especially preferably by not more than 10% of the maximum occurring clamping pressure.
24. The pressure sensor as claimed in claim 22, wherein:
- the axial clamping pressure on said first and second sealing elements does not, over temperature cycles between −40° C. and 150° C., sink below 0.8 MPa, preferably not below 0.9 MPa and especially preferably not below 1 MPa.
25. The pressure sensor as claimed in claim 22, wherein:
- said axially elastic element comprises a Belleville spring having the axial bearing surface.
26. The pressure sensor as claimed in claim 22, wherein:
- said axially elastic element is embodied as an axially flexible, corrugated tube membrane surrounding the media opening;
- said corrugated tube membrane has at a first axial end the axial bearing surface; and
- said corrugated tube membrane is connected pressure-tightly with said housing at a process connection and at a second end.
27. The pressure sensor as claimed in claim 15, further comprising:
- a compensation ring, which is axially clamped together with the pressure measuring cell, wherein:
- said compensation ring has a greater coefficient of thermal expansion than the material of said housing, and said pressure measuring cell has a smaller coefficient of thermal expansion than the material of said housing.
28. The pressure sensor as claimed in claim 27, wherein:
- said compensation ring comprises zinc, magnesium or aluminum.
Type: Application
Filed: Jul 16, 2004
Publication Date: Feb 8, 2007
Patent Grant number: 7448274
Applicant: Endress + Hauser GmbH+Co. KG (Maulburg)
Inventors: Ulfert Drewes (Mullheim), Karl Floegel (Schopfheim), Frank Hegner (Lorrach), Thomas Uehlin (Schopfheim), Rainer Martin (Efringen-Kirchen)
Application Number: 10/566,364
International Classification: G01L 7/00 (20060101);